There exists a need for technology directed to economical field monitoring of toxins, explosives, and chemical contaminants. The United States has, for example, implemented a number of regulatory acts to protect its ecologies and its citizens from environmental pollution, and these acts mandate the monitoring of various chemical contaminants and other biohazards. Field-portable equipment is needed to supplement lab-based instrumentation, especially hardware that can identify and quantify with high specificity particular species of interest. One of the most promising strategies for performing such narrowly targeted field assays is based on sensors that harness natural immune and protective responses of animals and humans to identify specific compounds. Examples of such approaches include fiber optic evanescent wave sensors and surface plasmon resonance sensors.
An electromagnetic wave that is reflected at a dielectric interface produces an exponentially decaying electric field within the material on the opposite side of the interface. At optical frequencies this is termed an evanescent wave, and at radio frequencies this phenomenon is often called a "skin effect." Although the penetration depth of the evanescent wave is typically a small fraction of a wave length, many compounds of interest are themselves small molecules. Detecting these compounds can be accomplished by coupling sensor molecules 100 such as antibodies to the surface of the core of an optical fiber 102, as shown in FIG. 1A. In one competitive assay technique, fluorescently tagged antigen 104, together with the sample to be tested, is exposed to the coating on the fiber, and the tagged antigen competes for antibody bonding sites with non-tagged analyte 106 in the test sample. The evanescent field produced by light 108 passing through the fiber 102 then excites the fluorophores into light emission 110, and the fiber itself conveniently acts as a return waveguide for the fluorescent signal. In this example, the strength of the fluorescent signal is inversely related to the analyte concentration in the test sample. Alternatively, a non-competitive technique, such as a sandwich assay, can be used, in which case the fluorescent signal is directly related to the analyte concentration in the test sample.
For surface plasmon resonance sensing, FIG. 1B shows a thin layer of metal 110, such as gold, applied to a core portion 112 of an optical fiber 114 from which the cladding 116 of the fiber has been partly removed. The evanescent electric field produced by light 118 passing through the fiber 114 excites surface plasmon waves 120 on the outer surface of the metal 110. When white light is passed through the fiber 114, the excitation of a surface plasmon wave causes a dip in the spectrum of the light passing through the fiber, with the dip occurring at a resonance wavelength which is a function of the complex indices of refraction of the fiber core, the metal layer, and the solution surrounding the fiber, as well as the incidence angle of the light. Light passing through the fiber 114 can be returned by a mirror 122, or can be through-put (in the absence of a mirror) for optical processing and analysis, as is well known to those skilled in the art. Any change in the index of refraction of the solution is detectable, and molecules binding to the surface of the metal 110 can then be detected if they have an index of refraction that is different from the bulk solution. Coating the metal layer 110 with sensor molecules (not shown), which react with target analytes within a sample solution, then allows detection of reactions (such as antigen-antibody reactions and reduction-oxidation reactions) on the surface of the metal.
Fiber optic evanescent wave sensors are the subject of a number of U.S. patents, including the following, the disclosures of each being incorporated herein by reference: U.S. Pat. No. 4,447,546, to Hirschfeld et al., entitled "Fluorescent Immunoassay Employing Optical Fiber in Capillary Tube"; U.S. Pat. No. 4,558,014, to Hirschfeld et al., entitled "Assay Apparatus and Method"; U.S. Pat. No. 4,582,809, to Block et al., entitled "Apparatus Including Optical Fiber for Fluorescence Immunoassay"; U.S. Pat. No. 4,654,532, to Hirschfeld, entitled "Apparatus for Improving the Numerical Aperture at the Input of a Fiber Optic Devices"; U.S. Pat. No. 4,716,121, to Block et al., entitled "Fluorescent Assays, Including Immunoassays, with Feature of Flowing Sample"; U.S. Pat. No. 4,909,990, to Block et al., entitled "Immunoassay Apparatus"; U.S. Pat. No. 5,242,797, to Hirschfeld, entitled "Nucleic Acid Assay Method"; U.S. Pat. No. 5,061,857, to Thompson et al., entitled "Waveguide-Binding Sensor for Use With Assays"; U.S. Pat. No. 5,430,813, Anderson et al., entitled "Mode-Matched, Combination Taper Fiber Optic Probe"; U.S. Pat. No. 5,152,962, to Lackie, entitled "Immunoassay Apparatus"; U.S. Pat. No. 5,290,398, to Feldman et al., entitled "Synthesis of Tapers for Fiber Optic Sensors"; and U.S. Pat. No. 5,399,866, to Feldman et al., entitled "Optical System for Detection of Signal in Fluorescent Immunoassay." Fiber optic surface plasmon resonance sensors are the subject of U.S. Pat. No. 5,359,681 to Jorgenson et al., entitled "Fiber Optic Sensor and Methods and Apparatus Relating Thereto," the disclosure of which is incorporated herein by reference.
For evanescent wave sensors, it is desirable to optimize the magnitude of the evanescent electric field as well as to optimize the optical properties of the return path for the detected fluorescence. The above-identified patents describe numerous optimization approaches, including attempts to match the numerical aperture of various system components and to improve system numerical aperture. Numerical aperture is a measure of the largest angle, relative to the optical axis of a system, that a ray of light can have and still pass through the system. Each component in an optical system will have its own unique limiting numerical aperture, and the maximum system numerical aperture will be determined by the system component having the lowest numerical aperture. The system numerical aperture is a key parameter in optical sensing since transferred power is typically proportional to its square. Good design practice and cost efficiencies require system components to have matching numerical apertures.
One well-known approach of matching numerical apertures employs tapered or cone-shaped waveguides. In addition to providing numerical aperture matching, tapering the active, analyte-sensitive portion of the optical fiber maintains a substantial fraction of the input light near the critical angle, thereby maintaining a high magnitude evanescent field. However, there is also a constant loss of light along the sensor fiber as the taper acts upon rays that are already only weakly guided and causes them to exceed the critical angle.
In order for white light to propagate in an optical fiber used in connection with a surface plasmon resonance sensor, the fiber must have a large enough diameter to support the longest wavelength of light. Also, a large diameter fiber propagates higher numerical aperture light, which makes it easier to excite surface plasmon waves in metal films of thicknesses readily fabricated by conventional processes. As a consequence, multi-mode fibers are used which propagate light over a range of angles. However, this range of angles results in a less distinct resonance effect, because each angle of propagation results in a different resonance wavelength.
FIG. 2A shows the theoretical resonance curves for various propagation angles relative to the optical axis of the fiber core, assuming a 55 nm thick layer of gold on a silica optical fiber core immersed in water. The overall resonance detected is a superposition of the resonance effects for each of the various angles of propagation. FIG. 2B shows the integration of individual theoretical resonance curves for propagation angles from 0 to 23.6 degrees, assuming a sine-squared distribution of optical power at the various propagation angles. The significant signal degradation associated with current approaches to surface plasmon resonance sensing is seen by comparing the resonance curve of FIG. 2B with the individual resonance curve of, for example, 23.6 degrees in FIG. 2A.
Although fiber optic evanescent wave and surface plasmon resonance sensors show great promise for use in field-portable assay equipment, those skilled in the art understand that the current technology is less than optimal in a number of respects, including those disadvantages identified above.